There is a possible issue with rolling the trackball... where you might slam your fingers into the spinner. Also, reaching that deep into the control panel can create physical strain in a very short period of time.
Also, your buttons are too far apart to easily be used. Typically, arcades placed them so close together, that it was often hard to turn the undersides button-nut. You want to be able to comfortably rest your fingers on two or the full three buttons... depending on your finger spread capability. (or at least, be able to easily shift from the center.. to either side with a minimal slide)
Use a few layers of cardboard to give thickness... will help with the mounting and strength. You could hot glue layers together.. and or also use some vertical supports too.
If you move your buttons closer.. you might have room to inject the spinner slightly diagonal to the right... of player 1s side.
This will allow you to use the joystick and or the spinner, and still use the buttons with the other hand.
Make sure to try to leave about 1ft diameter minimal of space for rolling the trackball. Nearby buttons are fairly low, so are not much of an issue. However, joysticks that are too close.. you can end up smashing your fingers / hand into.
A lot of people prefer the look of a symmetrical panel.. however, in doing so, it causes the joystick smash problem. Moving the trackball slightly to the left, towards player 1s sides buttons, usually solves the problem.
Because of this issue.. many have had to crank the sensitivity of the trackball really high.. so that they do not have to roll it much. However, this makes the games too easy, because you are reducing the depth of the analog spectrum. Meaning.. instead of a possible input value set of 1 to 100... now its 1 to 5. Vastly reducing possible outcomes and skill potentials.
Arcade Trackballs were made to be spun at high speeds, using nice long stroke spins.